It is well known in the industry that conventional methods for producing steel, are not suited to the production of stainless steel or other chromium containing steels. This is because conventional steel producing processes, such as the open hearth furnace or basic oxygen furnace, refine impurities from the steel by oxidation reactions. In the case of stainless steel, the chromium content therein would be oxidized to a great extent before suitable low levels of carbon and phosphorus could be achieved. Until quite recently therefore, most stainless steel produced in the United States has been produced by an electric arc furnace process primarily utilizing a charge of selected stainless steel and carbon steel scrap, and low-carbon ferrochromium. Since carbon and phosphorus are initially avoided in the charge, the metal can be easily refined without substantial chromium oxidation.
Although electric arc furnaces provide considerable flexibility and a high degree of control, arc furnace processes have the disadvantage of being costly and time consuming. In the past few years there has been a revolution in the stainless steel producing industry, as at least two new processes for its production have been utilized commercially, and others have been patented. For example, in at least one plant, stainless steel has been produced in a basic oxygen furnace utilizing less expensive high-carbon ferrochromium. The carbon is selectively oxidized therefrom without oxidation of chromium by maintaining an abnormally high temperature at which equilibrium conditions are such as to favor carbon oxidation. Because of the high temperatures necessary for this process, refractory life of the vessel is greatly shortened.
Another new process is the AOD (argon-oxygen decarburization) process which also permits the use of the lower cost high-carbon ferrochromium. In this process, selected scrap and high-carbon ferrochromium are first melted in an electric furnace and thereafter transferred to an AOD vessel where oxygen and argon are jointly blown through the metal via tuyeres through the side of the vessel. The argon in the blown gas serves to reduce the oxygen partial pressure and oxygen activity, and hence shift the carbon-chromium equilibrium to favor carbon oxidation. As lower carbon contents are achieved, the argon concentration is increased causing a further equilibrium shift and further carbon oxidation. Although this process has been widely adopted and does present considerable advantages over conventional electric furnace practices, it is still not as economical as would be hoped for because multiple vessels are required, because large volumes of rather expensive argon are required, and the subsurface oxygen injection does adversely affect refractory life in the AOD vessel.